Polymer electrolyte fuel cell

ABSTRACT

For a combination of a solid polymer electrolyte membrane  107 , catalytic layers  111  and  113  disposed on both sides of the solid polymer electrolyte membrane  107 , gas diffusion layers  112  and  114  disposed outside the catalytic layers  111  and  113 , and separators  103  and  104  disposed outside the gas diffusion layers  112  and  114 , the catalytic layer  113  to be cathode-sided includes a carbon carrier  117  composed of carbon having a mean lattice plane spacing d 002  of [002] planes calculated from an X-ray diffraction within a range of 0.343 nm to 0.358 nm, a crystallite size Lc within a range of 3 nm to 10 nm, and a specific surface area within a range of 200 m 2 /g to 300 m 2 /g, catalyst particles  115  containing platinum supported on the carbon carrier  117 , and an electrolyte  116 . According to the invention, a polymer electrolyte fuel cell is allowed to prevent a corroding deterioration of carbon carriers in the cathode catalytic layer in start and stop of the fuel cell, allowing for an enhanced stable output over a long term.

The present application is a divisional application of U.S. applicationSer. No. 11/791,679, filed May 25, 2007, which is the National Stage ofApplication No. PCT/JP2005/020083 filed on Nov. 1, 2005, which is basedupon and claims the benefit of priority from Japanese Patent ApplicationNos. 2004-340318 filed Nov. 25, 2004 and 2005-299289 filed Oct. 13,2005, the entire contents of all of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a polymer electrolyte fuel cell.

BACKGROUND ART

The fuel cell is an apparatus that converts chemical energy of a fueldirectly into electrical energy by no way through mechanical energy northermal energy, and is high in efficiency of power generation, and has,as the next-generation of power generating apparatus, great hopes putthereon.

As a fuel cell to be mounted on an automobile, a polymer electrolytefuel cell using an ion exchange membrane is watched. For the polymerelectrolyte fuel cell, the basic configuration and actions will bedescribed.

The polymer electrolyte fuel cell is configured as a complex cell thathas a plurality of laminated simplex cells (herein sometimes referred toas “single cells”) to be fundamental units for power generation.

Each of the single cells has an MEA (membrane electrode assembly) thathas a fuel electrode or positive electrode (hereinafter referred to as“anode”) and an oxidant electrode or negative electrode (hereinafterreferred to as “cathode”) interposed on both sides of a solid polymerelectrolyte membrane, respectively. Further, the single cell has ananode side separator and cathode side separator provided with gaschannels and cooling water channels, outside the anode and the cathode,respectively.

The anode has a catalytic layer outside the solid polymer electrolytemembrane, and has a fuel gas diffusion layer outside it. The cathodealso has a catalytic layer outside the solid polymer electrolytemembrane, and has an oxidant gas diffusion layer outside it.

In the polymer electrolyte fuel cell, a gaseous fuel (herein sometimesreferred to as “fuel gas”) containing hydrogen is supplied to the anode,where reactions of the following expression (1) occur in the catalyticlayer, and a gaseous oxidant (herein sometimes referred to as “oxidantgas”) containing oxygen is supplied to the cathode, where reactions ofthe following expression (2) occur in the catalytic layer.

H₂→2H⁺+2e ⁻  (1)

1/2O₂+2H⁺+2e ⁻→H₂O+Q (reaction heat)  (2)

Therefore, every single cell of the fuel cell apparently has a reactionof the following expression (3) progressing therein.

H₂+1/2O₂→H₂O+Q  (3)

This reaction accompanies a necessary electromotive force for movementof electron (e⁻), which can be taken outside in the form of electricalenergy.

As will be seen from the expression (1), the anode has hydrogen ions(protons) generated in the catalytic layer, which hydrogen ions move tothe gas diffusion layer in the cathode via proton exchange groups in thesolid polymer electrolyte membrane as a transmission medium. Protonexchange groups in the solid polymer electrolyte membrane have adecreased specific resistance as the electrolyte membrane has asaturating moisture content, acting as a proton-conductive electrolyte.Therefore, in order to keep the solid polymer electrolyte membrane in awater containing state, the reaction gas to be supplied to each singlecell is humidified in advance. In each single cell, the solid polymerelectrolyte membrane is thereby allowed for a suppressed evaporation ofthe moisture, with a resultant protection of the drying.

Further, as will be seen from the expression (3), the cathode has waterproduced in the catalytic layer as a power generating reaction isadvanced in the fuel cell, and the produced water flows downstream ineach single cell, together with oxidant gas. Therefore, by concurrentpresence of such water that has been contained in oxidant gas forhumidification of the solid polymer electrolyte membrane and such waterthat has been produced along with the power generating reaction, eachsingle cell may tend to have an increased content of moisture residingin the downstream region. Thus, there is a possibility that this regionmay be over-saturated, generating droplets, and impeding a favorablediffusion of oxidant gas.

To this point, the oxidant gas to be supplied may have a reduced contentof moisture for humidification to effect a decrease in total amount ofresidual moisture in the downstream region of each single cell, whichmay however be accompanied by a raised utilization of oxidant gas toincrease the efficiency of power generation, yet with the possibility ofproducing much water in the catalytic layer, causing an over-saturation,generating droplets.

Accordingly, in each single cell, the catalytic layer may have a carboncarrier of a porous planer or particle shape carrying a platinumcatalyst, and an intervenient electrolyte (e.g. PTTF, etc.) forprovision of a water repellency thereto, to thereby prompt drainingproduced water or condensed water.

In addition, as will he seen from the expression (1), the fuel cell hasin the startup a process of supplying a hydrogen gas as the fuel gas tothe anode, where the anode may have H₂ and residual air mixed in theupstream and the downstream, forming to the anode a local cell (with anupstream anode and a downstream cathode). Then, the solid polymerelectrolyte membrane neighboring the anode may have a deficient state ofhydrogen ion at the downstream, with a resultant gradient of hydrogenion concentration causing the solid polymer electrolyte membrane to havea lowered potential in the downstream. As a result, the solid polymerelectrolyte membrane may have an increased potential difference to thecatalytic layer at the cathode side, which may be accompanied byoccurrences of such a corrosion of carbon carriers as shown byexpressions (4) and (5) and such a melting of Pt as shown by anexpression (6), in the catalytic layer at the cathode side.

C+2H₂O→CO₂+4H⁺+4e ⁻  (4)

C+H₂O→CO₂+2H⁺+2e ⁻  (5)

Pt→Pt²⁺+2e ⁻  (6)

Such phenomina may occur in a start of the fuel cell, as well as in astop, with a yet accelerated tendency along repetition of start and stopoperations of the fuel cell. Thus, there is a possibility that theperformance of power generation may be reduced as the cell voltagedecreases.

With such points in view, for the cathode's catalytic layer as a factorto determine the performance of power generation, besides the drainage,it has been desired to suppress the catalyst's activity reduction due to(platinum) catalyst elution and carbon carrier corrosion.

For an enhanced anti-corrosiveness of a carbon carrier, Japanese PatentUnexamined Publication No. 2005-26174 has disclosed a cathode catalyticlayer, in which the carbon carrier has an increased degree ofgraphitization, and the specific surface area as well as the bulkdensity is set within a specified range.

On the other hand, for an enhanced activity of a platinum catalyst,Japanese Patent Unexamined Publication No. H6-150944 has disclosed anelectrode, in which the catalytic layer is double-layered and acatalytic layer at the solid polymer electrolyte membrane side has amore increased amount of platinum catalyst than that at the gasdiffusion layer side. Further, Japanese Patent Unexamined PublicationNo. H6-103982 has disclosed a fuel cell in which for a double-layeredcatalytic layer, in a catalytic layer at the solid polymer electrolytemembrane side, the amount of electrolyte is increased, or the amount ofplatinum catalyst is increased more than that in a catalytic layer atthe gas diffusion side of electrode. In addition, in Japanese PatentUnexamined Publication No. H11-312526, there has been disclosed even anelectrode in which, for a double-layered catalytic layer, the particlesize of metal catalyst in a catalytic layer at the gas diffusion side ofelectrode is set as greater as 1.5 times or more than the particle sizeof metal catalyst in a catalytic layer at the solid polymer electrolyteside.

DISCLOSURE OF INVENTION

However, the use of a carbon carrier with a high degree ofgraphitization, though giving an enhanced anti-corrosiveness,accompanies a tendency for the carbon carrier to have a decreasedspecific surface area. Thus, there is a possibility that catalystparticles supported on a carbon carrier may be aggregated, with areduced catalytic activity, causing the performance of power generationto be reduced.

Further, the catalytic layer which is double-layered by formingcatalytic layers with different particle diameters or support amounts ofcatalyst particles to be supported on carbon carriers gives an improvedpower generation characteristic. However, the catalytic layer providescarbon carriers in a catalytic layer at the solid polymer electrolytemembrane side with a reduced anti-corrosiveness in comparison with acatalytic layer at the gas diffusion layer side. Thus, a dispersion inanti-corrosiveness of carbon carriers in the double-layered catalyticlayer is occurred.

The present invention has been devised in view of the problemsdescribed.

To solve the problems, a polymer electrolyte fuel cell according to thepresent invention comprises: a solid polymer electrolyte membrane;catalytic layers disposed on both sides of the solid polymer electrolytemembrane; gas diffusion layers disposed outside the catalytic layers;and separators disposed outside the gas diffusion layers, wherein acathode-sided catalytic layer comprises a carbon carrier comprisingcarbon having a mean lattice plane spacing d₀₀₂ of [002] planescalculated from an X-ray diffraction within a range of 0.343 nm to 0.358nm, a crystallite size Lc within a range of 3 nm to 10 nm, and aspecific surface area within a range of 200 m²/g to 300 m²/g, catalystparticles containing platinum supported on the carbon carrier, and anelectrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a fuel cell stack with a polymerelectrolyte fuel cell according to an embodiment of the presentinvention.

FIG. 2 is a schematic sectional view of the fuel cell stack shown inFIG. 1.

FIG. 3 is a sectional view of a single cell shown in FIG. 2.

FIG. 4 is a partial enlarged sectional view of a periphery of a cathodeshown in FIG. 3.

FIG. 5 is an enlarged sectional view of a carbon carrier in a cathodecatalytic layer shown in FIG. 4.

FIG. 6 is an enlarged sectional view of a cathode catalytic layeraccording to a second embodiment of the present invention.

FIG. 7 is a diagram describing a potential distribution in a vicinity ofa cathode in a single cell, in a start of the fuel cell.

FIG. 8 is an enlarged sectional view of a cathode catalytic layeraccording to a third embodiment of the present invention.

FIG. 9 is a diagram illustrating a potential distribution in a sectionaldirection in a vicinity of the cathode, in power generation of the fuelcell.

FIG. 10 is an enlarged sectional view of a cathode catalytic layeraccording to a fourth embodiment of the present invention.

FIG. 11 is a sectional view of a membrane electrode assembly accordingto a fifth embodiment of the present invention.

FIG. 12 is an enlarged sectional view of a double-layered catalyticlayer shown in FIG. 11.

FIG. 13 is a diagram describing a movement of proton in a solid polymerelectrolyte membrane in introduction of a fuel gas (hydrogen gas).

FIG. 14 is a diagram illustrating a potential distribution in asectional direction in a vicinity of a cathode A in a region opposing avicinity of an upstream of the fuel gas.

FIG. 15 is a sectional view of a membrane electrode assembly accordingto a sixth embodiment of the present invention.

FIG. 16 is a diagram describing a movement of proton in a solid polymerelectrolyte membrane in power generation of the fuel cell.

FIG. 17 is a diagram illustrating a potential distribution in asectional direction in a vicinity B of a cathode in a vicinity of adownstream of an oxidant gas.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described below a polymer electrolyte fuel cell accordingto an embodiment of the present invention, with reference to theaccompanying drawings.

First Embodiment

FIG. 1 is a perspective view of a fuel cell stack 100 with a polymerelectrolyte fuel cell according to an embodiment of the presentinvention, and FIG. 2 schematically shows a partial section of the fuelcell stack 100. The fuel cell stack 100 is configured as a complex cellwith a plurality of laminated single cells 101. The single cells 101have an anode side separator 103 and a cathode side separator 104residing on both sides of a membrane electrode assembly 102. The fuelcell stack 100 has end flanges 105 a and 105 h disposed at both ends ofthe plurality of laminated single cells 101, and is configured byfastening the outer peripheral parts with fastening bolts 106.

In addition, FIG. 3 shows a section of a single cell 101. This singlecell 101 is configured in the form of a membrane electrode assembly 110with an anode 108 and a cathode 109 disposed on both sides of a solidpolymer electrolyte membrane 107, respectively, and an anode sideseparator 103 and a cathode side separator 104 disposed outside theanode 108 and the cathode 109.

The anode 108 has a catalytic layer 111 on an outside of the solidpolymer electrolyte membrane 107, and has a fuel gas diffusion layer 112on an outside thereof. Also the cathode 109 has a catalytic layer 113 onan outside of the solid polymer electrolyte membrane 107, and has a gasdiffusion layer 114 on an outside thereof.

Referring now to FIG. 4, this shows a partial enlarged section of thecathode 109 shown in FIG. 3. The cathode 109 has the cathode catalyticlayer 113 and the gas diffusion layer 114 formed in order from the solidpolymer electrolyte membrane 107 side. The cathode catalytic layer 113has carbon carriers 117 supporting pluralities of catalyst particles 115containing platinum (Pt) or platinum alloy thereon, as shown in FIG. 5,and is made in the form of carbon carriers 117 bonded by an electrolyte116.

According this embodiment of the invention, carbon carriers 117 in thecathode catalytic layer 113 have a mean lattice plane spacing d₀₀₂ of[002] planes calculated from an X-ray diffraction within a range of0.343 nm to 0.358 nm, a crystallite size Lc within a range of 3 nm to 10nm, and a specific surface area within a range of 200 m²/g to 300 m²/g.

To specify the degree of graphitization of carbon carriers 117, first,for [002] planes calculated from an X-ray diffraction, the mean latticeplane spacing d₀₀₂ is defined within a range of 0.343 nm to 0.358 nm. Inthis respect, if the degree of graphitization of carbon carriers 117becomes higher (if the mean lattice plane spacing d₀₀₂ becomes smallerthan 0.343 nm), then carbon carriers 117 have a reduced specific surfacearea, with a resultant aggregation of catalyst particles 115 accompaniedby enlargement of catalyst particles 115, causing an unfavorabledispersion of particles 115 being metal, so that catalyst particles 115may have a reduced catalytic activity on oxygen. To the contrary, if thedegree of graphitization of carbon carriers 117 becomes lower (if themean lattice plane spacing d₀₀₂ of [002] planes exceeds 0.358 nm), thencarbon carriers 117 tend to be corroded in start and stop of the fuelcell, so that the fuel cell may have a greatly reduced output in thestart during a long-term service of the fuel cell.

Further, for carbon carriers 117, the crystallite size Lc is definedwithin a range of 3 nm to 10 nm. In this respect, if the crystallitesize Lc of carbon carriers 117 becomes smaller than 3 nm, the degree ofgraphitization becomes too low for carbon carriers 117 to haveanti-corrosiveness. To the contrary, if the crystallite size Lc ofcarbon carriers 117 exceeds 10 nm, then the degree of graphitizationbecomes high, and carbon carriers 117 have a remarkably reduced specificsurface area, with a resultant aggregation of catalyst particles 115giving an enlarged particle size, causing an unfavorable dispersion ofcatalyst particles 115, so that catalyst particles 115 may have areduced catalytic activity on oxygen.

In addition, for carbon carriers 117, the specific surface area isdefined within a range of 200 m²/g to 300 m²/g. In this respect, if thespecific surface area of carbon carriers 117 becomes smaller than 200m²/g, then in the case of an increased support amount of catalystparticles 115, the dispersion of catalyst particles 115 may be reduced.To the contrary, if the specific surface area of carbon carriers 117exceeds 300 m²/g, then with a reduced anti-corrosiveness, and with aninsufficient coverage of carbon carriers 117 by electrolyte 116, theamount of catalyst particles 115 unused for the reducing reaction ofoxygen may be increased. Contrary thereto, by specification for thespecific surface area of carbon carriers 9 to be within 200 m²/g to 300m²/g, carbon carriers 117 are allowed to have pluralities of catalystparticles 115 uniformly dispersed and supported thereon, having catalystparticles 115 covered with electrolyte 116, so that the grain growth ofcatalyst particles 115 supported on carbon carriers 117 can besuppressed, allowing for a stable reaction activity of electrode over along term.

For use as carbon carriers 117 meeting the above-noted conditions, itmay be preferable to employ a carbon black having a mean particle sizewithin a range of 12 nm to 25 nm, a bulk density within a range of 0.09g/cm³ to 0.13 g/cm³, and an electrical resistivity within a range of0.27 Ωcm to 0.33 Ωcm.

Further, for use as carbon carriers 117, it may be preferable to employan acetylene black having a mean lattice plane spacing d₀₀₂ of [002]planes within a range of 0.343 nm to 0.355 nm, a crystallite size Lcwithin a range of 3 nm to 9 nm, a specific surface area within a rangeof 200 m²/g to 280 m²/g, a mean particle size within a range of 16 nm to20 nm, a bulk density within a range of 0.10 g/cm³ to 0.12 g/cm³, and anelectrical resistivity within a range of 0.29 to 0.32 Ωcm.

In addition, catalyst particles 115 may preferably occupy a proportionwithin a range of 30% to 70% in a mass conversion with respect to atotal amount of catalyst particles 115 and carbon carriers 117 residingin the cathode catalytic layer 113, as shown by expression 1.

Proportion of catalyst particles=mass of catalyst particles/(mass ofcatalyst particles+mass of carbon carriers)×100  (7)

In this respect, if the proportion of catalyst particles 115 becomessmaller than 30%, then with a reduced support amount of catalystparticles 115, the catalytic activity may go down, and to the contrary,if the proportion of catalyst particles 115 exceeds 70%, then thecatalytic activity may be unsuccessfully increased for the increase incost.

Further, carbon carriers 117 supporting catalyst particles 115 thereonmay preferably have a specific surface area within a range of 60 m²/g to200 m²/g. In this respect, if the specific surface area of carboncarriers 117 supporting catalyst particles 115 thereon becomes smallerthan 60 m²/g, then with a reduced catalytic active site, the activity ofcatalyst may be reduced. To the contrary, if the specific surface areaof carbon carriers 117 supporting catalyst particles 115 thereon exceeds200 m²/g, then with an insufficient coverage of carbon carriers 117 byelectrolyte 116, the amount of catalyst particles 115 unused for thereducing reaction of oxygen may be increased.

In addition, the solid polymer electrolyte membrane 107 and electrolyte116 in the cathode catalytic layer 113 may preferably be composed ofperfluorocarbon polymers having sulfonic acid groups.

Further, the cathode catalytic layer 113 may preferably have an averagethickness ranging 6 μm to 15 μm. If the cathode catalytic layer 113becomes thicker, then oxygen gas may be unsuccessfully diffused tocatalyst particles 115 supported on carbon carriers 117 covered byelectrolyte 116, and the cathode catalytic layer 113 may have residualwater (condensed water of supplied water for humidification, andproduced water) therein, with a resultant tendency to provide a reducedoutput due to a flooding in regions of high current density by residualwater. As a result, carbon carriers 117 may fail to haveanti-corrosiveness in start and stop of the fuel cell. To the contrary,if the cathode catalytic layer 113 becomes thinner, then withinsufficient secured intervals of time for contact between catalystparticles 115 and oxygen gas, and with a decreased catalytic activity ofoxygen, the fuel cell may have a greatly reduced output in powergeneration when used over a long term.

Concurrently with implementation of a specified thickness of the cathodecatalytic layer 113, carbon carriers 117 supporting catalyst particles115 thereon may preferably have a proportion of existence within a rangeof 50% to 80% with respect to a total mass in which carbon carriers 117supporting catalyst particles 115 thereon and electrolyte 116 in thecathode catalytic layer 113 are summed up. If carbon carriers 117supporting catalyst particles 115 thereon become smaller than a 50%,then the catalytic activity may be reduced. To the contrary, if carboncarriers 117 supporting catalyst particles 115 thereon exceed a 80%,then the quantity of electrolyte 116 may be too small to cover carboncarriers 117.

On the other hand, the anode catalytic layer 111 may preferably have anaverage thickness ranging 2 μm to 10 μm. If the thickness of anodecatalytic layer 111 exceeds 10 μm, then the anode catalytic layer 111may have an increased amount of residual water therein, with a resultantdecrease in the amount of water to be diffused back from the cathodecatalytic layer 113 through the solid polymer electrolyte membrane 107,and the cathode catalytic layer 113 may have a maintained amount ofwater retained therein. As a result, carbon carriers 117 in the cathodecatalytic layer 113 may have a reduced anti-corrosiveness in start andstop of the fuel cell. To the contrary, if the anode catalytic layer 111becomes smaller than a 2 μm, then with insufficient secured intervals oftime for contact between catalyst particles and hydrogen gas, and with adecreased catalytic activity of hydrogen, as well as with an increasedfrequency of cycles between humidification and drying at the solidpolymer electrolyte membrane 107 contacting the anode catalytic layer111, the solid polymer electrolyte membrane 107 may have a reduceddurability. Further, when the fuel cell is put in service over a longterm, an output of power generation initially achieved in the operationmay be greatly reduced.

Concurrently with implementation of a specified thickness of the anodecatalytic layer 111, carbon carriers supporting catalyst particlesthereon may preferably have a proportion of existence within a range of50% to 80% with respect to a total mass in which carbon carrierssupporting catalyst particles thereon and electrolyte are summed up. Inthe anode catalytic layer 111, if carbon carriers supporting catalystparticles thereon become smaller than a 50%, then the catalytic activitymay go down. To the contrary, if carbon carriers supporting catalystparticles thereon exceed a 80% by weight, then carbon carriers may beunsuccessfully covered with electrolyte.

In addition, the anode catalytic layer 111 may preferably have anaverage thickness (Ya) thinner than an average thickness (Ye) of thecathode catalytic layer 113, and meet a relationship between Ya and Yc,such that Ya/Yc=0.1 to 0.6. If Ya/Yc becomes smaller than 0.1, then inthe anode catalytic layer 111, with insufficient intervals of time forcontact between catalyst particles and hydrogen gas, and with adecreased catalytic activity of hydrogen, or with an increased frequencyof cycles between humidification and drying at the solid polymerelectrolyte membrane 107 contacting the anode catalytic layer 111, thesolid polymer electrolyte membrane 107 may have a reduced durability. IfYa/Yc exceeds 0.6, then with a decrease in the amount of water to bediffused back from the cathode catalytic layer 113 through the solidpolymer electrolyte membrane 107, the cathode catalytic layer 113 mayhave an insufficiently reduced amount of water retained therein, so thatcarbon carriers in the cathode catalytic layer 113 may have a reducedanti-corrosiveness in start and stop of the fuel cell. As a result, in along-term service of polymer electrolyte fuel cell, a cell outputinitially achieved in the operation may be greatly reduced. In addition,it is turned out that by provision of the anode catalytic layer 111 withan average thickness thinner than an average thickness of the cathodecatalytic layer 113, when air-purging the anode in a start and a stop ofthe fuel cell, the amount of moisture in the cathode catalytic layer 113can be reduced with ease, allowing for a facilitated drying. As thecathode catalytic layer 113 has a decreased amount of moisture,migration of water takes place from the side of solid polymerelectrolyte membrane 107 having much moisture to the side of anodecatalytic layer 111, and concurrently, water in cathode catalytic layer113 vicinal to the solid polymer electrolyte membrane 107 or invicinities of interfacial planes of both layers 111 and 113 moves to theside of solid polymer electrolyte membrane 107. Therefore, the cathodecatalytic layer 113 has a decreased amount of retained water or residualmoisture, with an improved drainage, so that, all the way from aninitial phase of start to a post-endurance of the fuel cell, the gasdiffusion and draining characteristics can be kept from turndown, with aresultant enhancement in power generation performance encompassing froma low current density up to a high current density, allowing for amaintained durability and implementation of an elongated service life.

Further, catalyst particles in the anode catalytic layer 111 and thecathode catalytic layer 113 may preferably be platinum (Pt) or aplatinum alloy containing platinum (Pt), in view of power generationperformance (hydrogen oxidation activity at the anode and oxygenreduction activity at the cathode) and endurance (suppression of Pt oradditive component elution due to potential variation), and the platinumalloy may preferably contain a metal selected from among ruthenium (Ru),rhodium (Rh), palladium (Pd), iridium (Ir), osmium (Os), chromium (Cr),cobalt (Co), and nickel (Ni).

In addition, the mixing ratio of platinum and metal in the platinumalloy may preferably be set within a range of 3/1 to 5/1 in mole ratio(platinum/metal), in view of power generation performance and endurance.This is because, if the mole ratio (platinum/metal) exceeds 1/3, thesolid solution of metal added to platinum may become insufficient, witha resultant elution of metal in potential variation, with a reducedendurance. To the contrary, if the mole ratio (platinum/metal) issmaller than 5/1, the variation in potential of platinum due to addedcomponent may become insufficient, with an unsuccessful enhancement ofcatalytic activity.

In addition, carbon carriers included in the anode catalytic layer 111may preferably be low crystalline (amorphous material), and have aspecific surface area within a range of 300 m²/g to 1,500 m²/g. Likethis, provision of carbon carriers in the anode catalytic layer 111 withan enhanced hydrophilic property in comparison with carbon carriers 117in the cathode catalytic layer 113 enables a promoted migration (backdiffusion) of water from the side of solid polymer electrolyte membrane107 where the amount of moisture is high to the anode side where it islow. Concurrently therewith, moisture in cathode vicinal to the solidpolymer electrolyte membrane 107, i.e., in vicinities of an interfacialplane between cathode catalytic layer 113 and solid polymer electrolytemembrane 107 moves to the side of solid polymer electrolyte membrane107, with a resultant enhancement of drainage. Therefore, all the wayfrom an initial phase of start to a post-endurance of the fuel cell, thegas diffusion and draining characteristics can be kept from turndown,with a resultant enhancement in power generation performanceencompassing from a low current density up to a high current density,allowing for a maintained durability and implementation of an elongatedservice life.

Description will be made by employing specific examples of embodiment,without intended restriction to the illustrative examples.

Embodiment Example 1 Fabrication of Anode Catalytic Layer

First, a carbon black (Ketjen Black International Ltd. make Ketjenhlack™EC, specific surface area BET=800 m²/g, amorphous carbon) was preparedby 40 g, and 400 g of dinitro diammineplatinum solution (Ptconcentration 1.0%) was added to that carbon black, which was stirredfor one hour. Thereafter, 50 g of methanol was mixed thereto as areducing agent, which was stirred for one hour, and then heated up to80° C., stirred at 80° C. for six hours, and let to temperature-fall byone hour down to a room temperature. After a filtering of deposit,obtained solids were dried under a reduced pressure at 85° C. for 12hours, and crushed in a mortar, obtaining carbon carriers supportingthereon 50 mass-% in Pt support concentration of Pt particles having anaverage particle size of 2.6 nm.

Next, to obtained carbon carriers having Pt particles supported thereon,5 times their mass of purified water was added, and after five minutesof de-foaming operation under reduced pressure, 0.5 times their mass ofn-propyl alcohol was added, and then a solution (Du Pont Ltd. make)containing 20 wt % Nafion® to be an electrolyte was added. Theelectrolyte used in the solution had been prepared with a ratio of massof solids to mass of carbon carriers (carbon (carbon)/ionomer(electrolyte)) set to 1.0/0.9.

An obtained mixture as a slurry was dispersed with an ultrasonichomogenizer, and by application of a de-foaming operation under reducedpressure, a catalyst slurry was prepared. Prepared catalyst slurry wasprinted in a screen printing method on one side of apolytetrafluoroethylene seat, by an amount corresponding to a desirablethickness, and dried at 60° C. for 24 hours. By the screen printingmethod, anode catalytic layers were prepared, which had a size of 5 cm×5cm. Further, an adjustment had been made for the layer coated on thepolytetrafluoroethylene seat to have a Pt amount of 0.2 mg/cm² (for theanode catalytic layer to have an average thickness of 6 μm).[Fabrication of Cathode Catalytic Layer]

First, a high crystallinity carbon (Denki Kagaku Kogyo Ltd. makeacetylene black CA-200) was prepared, with a specific surface area BETof 216 m²/g, a mean lattice plane spacing d₀₀₂ of 0.343 nm, and acrystallite size Lc of 8.3 nm.

Next, 4.0 g of high crystallinity carbon was added to 400 g of dinitrodiammineplatinum solution (Pt concentration 1.0%), which was stirred forone hour, and thereafter, 50 g of formic acid was additionally mixedthereto as a reducing agent, which was stirred for one hour. Thereafter,it was heated up to 40° C. by 30 minutes, and stirred at 40° C. for sixhours, and thereafter, heated up to 60° C. by 30 minutes, andadditionally stirred at 60° C. for six hours, and was let totemperature-fall by one hour down to a room temperature. After afiltering of deposit, obtained solids were dried under a reducedpressure at 85° C. for 12 hours, and crushed in a mortar, obtainingcarbon carriers supporting thereon 50 mass-% in Pt support concentrationof Pt particles having an average particle size of 4.8 nm.

Next, to carbon carriers having Pt particles supported thereon, 5 timestheir mass of purified water was added, and after five minutes ofde-foaming operation under reduced pressure, 0.5 times their mass ofn-propyl alcohol was added. Thereafter, a solution (Du Pont Ltd. makewith 20 wt % Nafion®) containing a proton-conductive polymer electrolytewas additionally added. The content of polymer electrolyte used in thesolution had been prepared with a ratio of mass of solids to mass ofcarbon in catalysts of cathode electrode set for carbon/ionomer=1.0/0.9.

A mixture obtained as a slurry was dispersed with an ultrasonichomogenizer, and by application of a de-foaming operation under reducedpressure, a catalyst slurry was prepared. This catalyst slurry wasprinted in a screen printing method on one side of apolytetrafluoroethylene seat, by an amount corresponding to a desirablethickness, and dried at 60° C. for 24 hours. By the screen printingmethod, cathode catalytic layers were prepared, which had a size of 5cm×5 cm. Further, an adjustment had been made for the layer coated onthe polytetrafluoroethylene seat to have a Pt amount of 0.4 mg/cm² (forthe cathode catalytic layer to have an average thickness of 12 μm).

[Fabrication of Membrane Electrode Assembly]

Using Nafion™111 (membrane thickness 25 μm) as a solid polymerelectrolyte membrane, the solid polymer electrolyte membrane(Nafion™111) was superposed on an anode catalytic layer formed on apolytetrafluoroethylene seat, and in addition, a cathode catalytic layerformed on a polytetrafluoroethylene seat was superposed in a laminatingmanner. Subsequently, after 10 minutes of hot pressing at 130° C. under2.0 MPa, the polytetrafluoroethylene seats was peeled off, to provide amembrane electrode assembly.

On the solid polymer electrolyte membrane, there was a transferredcathode catalytic layer, which had a thickness of about 12 μm, and a Ptsupport amount of 0.4 mg per 1 cm² apparent electrode area, while theelectrode area was 25 cm². An anode catalytic layer had a thickness ofabout 6 μm, and a Pt support amount of 0.2 mg per 1 cm² apparentelectrode area, while the electrode area was 25 cm².

For the membrane electrode assembly obtained, a performance wasevaluated as follows.

On both sides of the membrane electrode assembly, gas diffusion layersof a carbon paper (size: 6.0 cm×5.5 cm, thickness: 320 μm) andgas-separating separators formed with gas channels were arranged,respectively, which was sandwiched by gold-plated stainless steelelectricity collectors to provide a unit sell for evaluation.

To the single cell for the evaluation, hydrogen gas was supplied as afuel at the anode side, and air was supplied as an oxidant at thecathode side. Both gases of atmospheric air and hydrogen gas hadatmospheric pressures as their supply pressures, the hydrogen gas being58.6° C. in temperature and 60% in relative humidity, and the air, 54.8°C. in temperature and 50° A) in relative humidity, while the celltemperature was 70° C. Further, the rate of use of hydrogen was set to67%, and the rate of use of air was set to 40%. Under this condition,electric power was generated with a current density of 1.0 A/cm², whenthe cell voltage was measured as an initial cell voltage.

After a subsequent power generation of 60 seconds, the power generationwas stopped. After the stop of power generation, the supply of hydrogenas well as that of air was stopped, and for a displacement of hydrogengas, air was supplied by 0.1 L/min at the anode side, which was followedby a waiting interval of 50 seconds. Then, at the anode side, hydrogengas was supplied by 0.05 L/min. Thereafter, under like conditions to theforegoing, hydrogen gas was supplied at the anode side, and air, at thecathode side, and power generation was again performed with a currentdensity of 1.0 A/cm² for 60 seconds. Further, in this time, the loadcurrent was increased from 0 A/cm² to 1 A/cm² by 30 seconds. Exercisingsuch start and stop of power generation, cell voltages were measured forthe evaluation of power generation performance. More specifically, whena cell voltage of 0.45V was given by a current density of 1.0 A/cm², thenumber of cycles was taken as an evaluation value of durability.

Embodiment Example 2

For an embodiment example 2, a single cell for evaluation was fabricatedby using like method to the embodiment example 1, subject to a change ofcarbon carriers supporting catalyst particles thereon in a cathodecatalytic layer.

First, a high crystallinity carbon (Denki Kagaku Kogyo Ltd. makeacetylene black CA-250) was prepared, with a specific surface area BETof 264 m²/g, a mean lattice plane spacing d₀₀₂ of 0.355 nm, and acrystallite size Lc of 3.6 nm.

To 4.0 g of this high crystallinity carbon, 400 g of dinitrodiammineplatinum solution (Pt concentration 1.0%) was added, which wasstirred for one hour. In addition, 50 g of formic acid was mixed theretoas a reducing agent, which was stirred for one hour, and thereafter, itwas heated up to 40° C. by 30 minutes, and stirred at 40° C. for sixhours. After a heating up to 60° C. by 30 minutes followed by anadditional stirring at 60° C. for six hours, it was let totemperature-fall by one hour down to a room temperature. After afiltering of deposit, obtained solids were dried under a reducedpressure at 85° C. for 12 hours, and crushed in a mortar, obtainingcarbon carriers supporting thereon Pt particles with an average particlesize of 3.5 nm, and a Pt support concentration of 50 mass-%.

Embodiment Example 3

For an embodiment example 3, a single cell for evaluation was fabricatedby using like method to the embodiment example 1, subject to a change ofcarbon carriers supporting catalyst particles thereon in a cathodecatalytic layer.

First, a graphitized Ketjenblack was prepared with a specific surfacearea BET of 200 m²/g, a mean lattice plane spacing d₀₀₂ of 0.343 nm, anda crystallite size Lc of 3.9 nm.

To 4.0 g of this graphitized Ketjenblack, 400 g of dinitrodiammineplatinum solution (Pt concentration 1.0%) was added, which wasstirred for one hour. In addition, 50 g of formic acid was mixed theretoas a reducing agent, which was stirred for one hour, and thereafter, itwas heated up to 40° C. by 30 minutes, and stirred at 40° C. for sixhours. After a heating up to 60° C. by 30 minutes followed by anadditional stirring at 60° C. for six hours, it was let totemperature-fall by one hour down to a room temperature. After afiltering of deposit, obtained solids were dried under a reducedpressure at 85° C. for 12 hours, and crushed in a mortar, obtainingcarbon carriers supporting thereon Pt particles with an average particlesize of 5.5 nm, and a Pt support concentration of 50 mass-%.

Comparative Example 1

For a comparative example 1, an MEA was fabricated, like the embodimentexample 1, subject to an alteration of carbon carriers in a cathodecatalytic layer to a Ketjen Black International Ltd. make Ketjenblack™EC.

Embodiment Examples 4 to 10, and Comparative Examples 2 to 3

For embodiment examples 4 to 10, the MEA was configured with adouble-layered cathode catalytic layer 1. Embodiment examples 4 to 6 aswell as comparative example 2 and comparative example 3 correspond to asecond embodiment to be described later, embodiment example 7, to athird embodiment, embodiment examples 8, to a fourth embodiment, andembodiment example 9, to a fifth embodiment. For any of them, the MEAwas fabricated by using like method to the embodiment example 1.

For the embodiment examples and comparative examples, the properties ofemployed carbon carriers in cathode and anode are listed in Table 1, andthe evaluation results of power generation performance are listed inTable 2.

TABLE 1 Cathode Catalytic Layers Carbon carriers Anode Catalytic LayersMean latice Specific Ave. Pt Carbon carriers plane Crystallite surfaceparticle Average Specific Ave. Pt Average spacings sizes areas sizesthickness surface areas particle sizes thickness Kinds d002 [nm] Lc [nm][m²/g] [nm] [μm] Kinds [m²/g] [nm] [μm] Emb. Acetylene black (CP200)0.343 8.3 216 4.8 12.0 Ketjenblack 800 2.6 6.0 Ex. 1 Emb. Acetyleneblack (CP250) 0.355 3.6 264 3.5 12.0 Ketjenblack 800 2.6 6.0 Ex. 2 Emb.Graphitized Ketjenblack 0.343 3.9 200 5.5 12.0 Ketjenblack 800 2.6 6.0Ex. 3 Emb. Ketjenblack (1st layer) — — 800 2.6 6.0 Ketjenblack 800 2.66.0 Ex. 4 Acetylene black (CP200) 0.343 8.3 216 4.8 6.0 Ketjenblack 8002.6 6.0 (2nd layer) Emb. Ketjenblack (1st layer) — — 800 2.6 6.0Ketjenblack 800 2.6 6.0 Ex. 5 Acetylene black (CP250) 0.355 3.6 264 3.56.0 Ketjenblack 800 2.6 6.0 (2nd layer) Emb. Ketjenblack (1st layer) — —800 2.6 6.0 Ketjenblack 800 2.6 6.0 Ex. 6 Graphitized Ketjenblack 0.3433.9 200 5.5 6.0 Ketjenblack 800 2.6 6.0 (2ndt layer) Emb. Ketjenblack(1st layer) — — 800 2.6 6.0 Ketjenblack 800 2.6 6.0 Ex. 7 GraphitizedKetjenblack 0.343 3.9 200 5.5 6.0 Ketjenblack 800 2.6 6.0 (2ndt layer)Emb. Ketjenblack (1st layer) — — 800 7.3 (PtCo) 6.0 Ketjenblack 800 2.66.0 Ex. 8 Graphitized Ketjenblack 0.343 3.9 200 5.5 10.0 Ketjenblack 8002.6 6.0 (2ndt layer) Emb. Ketjenblack (1st layer) — — 800 2.6 6.0Ketjenblack 800 2.6 6.0 Ex. 9 Acetylene black (CP250) 0.355 3.6 264 3.56.0 Ketjenblack 800 2.6 6.0 (2nd layer) Emb. Ketjenblack (1st layer) — —800 2.6 6.0 Ketjenblack 800 2.6 6.0 Ex. 10 Graphitized Ketjenblack 0.3433.9 200 5.5 6.0 Ketjenblack 800 2.6 6.0 (2ndt layer) Comp. Ketjenblack —— 800 2.6 6.0 Ketjenblack 800 2.6 12.0 Ex. 1 Comp. Ketjenblack (1stlayer) — — 800 2.6 6.0 Ketjenblack 800 2.6 6.0 Ex. 2 Vulcan (2nd layer)— — 275 3.4 6.0 Comp. Ketjenblack (1st layer) — — 800 2.6 6.0Ketjenblack 800 2.6 6.0 Ex. 3 Black pearl (2nd layer) — — 1550 2.4 6.0

TABLE 2 Durability (cycles) Emb. Ex. 1 1,760 Emb. Ex. 2 1,630 Emb. Ex. 31,950 Emb. Ex. 4 1,870 Emb. Ex. 5 1,920 Emb. Ex. 6 2,050 Emb. Ex. 72,190 Emb. Ex. 8 2,250 Emb. Ex. 9 2,280 Emb. Ex. 10 2,310 Comp. Ex. 1450 Comp. Ex. 2 650 Comp. Ex. 3 540

As shown in the Table 2, for MEA's fabricated in the embodiment examplesand the comparative examples, their start-stop cycles were compared,with a resultant verification of a higher start-stop duration, as theMEA employed in its cathode catalytic layer an electrode catalyst with ahigher crystalliinty carbon carrier supporting a platinum catalystcomponent thereon.

Second Embodiment Embodiment Examples 4 to 6, and Comparative Examples 2to 3

For a second embodiment, the cathode catalytic layer is double-layered,and respective catalytic layers have their carbon carriers different inmaterial. FIG. 6 is an enlarged sectional view of a catalytic layer 10in a cathode. The cathode catalytic layer 10 has a double-layeredstructure with a first catalytic layer 12 and a second catalytic layer13, the first catalytic layer 12 neighboring a solid polymer electrolytemembrane 2.

The first catalytic layer 12 is composed of an amorphous carbon 15supporting platinum (Pt) particles 14 thereon, with interveningelectrolyte 16, having a support amount of Pt particles 14 set to 0.2mg/cm², where the support amount of Pt particles means the amount ofsupported Pt particles per unit area. On the other hand, the secondcatalytic layer 13 is composed of a high-crystallinity carbon 18supporting Pt particles 17 thereon, with intervening electrolyte 19,having a support amount of Pt particles 17 set to 0.2 mg/cm² like thefirst catalytic layer 12.

It is noted that the combination of carbon carriers using an amorphouscarbon 17, such as a Ketjenblack, and a high-crystallinity carbon 18,such as an acetylene black or graphitized Ketjenblack, is not limitedthereto, providing that carbon carriers in the second catalytic layer 13are excellent in oxidizing (corroding) potential or anti-corrosiveness,in comparison with carbon carriers in the first catalytic layer 12.

FIG. 7 is a diagram describing potential distributions in a vicinity ofa cathode 4 of a single cell 1 in a start of a fuel cell.

When the fuel cell is generating electric power, protons (H⁺) run fromthe anode side of the solid polymer electrolyte membrane 2 toward thecathode side, and for electrolyte of the solid polymer electrolytemembrane 2 as well as electrolyte 16 in the catalytic layer 12 andelectrolyte 19 in the catalytic layer 13, the electric potentialsdecrease along flux of protons (H⁺). In this situation, at the cathode,the second catalytic layer 13 has a lower electrolyte potential incomparison with the first catalytic layer 12. This phenomenon is notrestrictive to the start of fuel cell, and takes place when protons (H⁺)move from the anode toward the cathode. Further, for the first catalyticlayer 12 and the second catalytic layer 13 electrochemically contactingeach other, the movements of electrons are very fast, so that theirelectrode potentials are equivalent. Thus, the first catalytic layer 12and the second catalytic layer 13 have an equal electrode potential, andthe second catalytic layer 13 has a lower electrolyte potential than thefirst catalytic layer 12, whereby for voltages (potential differences)imposed across the catalytic layers 12 and 13, the voltage (potentialdifference) V2 is greater than V1, as shown in FIG. 7. For un-corrosionof carbon carrier, this has an increased tendency in particular whenexposed to high voltages, and carbon carriers have a higher tendency tocorrode in the second catalytic layer 13 than in the first catalyticlayer 12.

According to the present embodiment using in a second catalytic layer ahigh-crystallinity carbon such as an acetylene black or graphitizedKetjenblack, the second catalytic layer has an increased oxidizingpotential in comparison with a first catalytic layer using an amorphouscarbon, allowing for an enhanced anti-corrosiveness over an entirety ofthe cathode. Further, when compared with a case using high-anticorrosivecarbon carriers in both first catalytic layer and second catalyticlayer, the three-phased interfaces are likely to be optimized, allowingfor a raised voltage, as an advantage.

Third Embodiment Embodiment Example 7

For a third embodiment, the cathode catalytic layer is double-layered,and respective catalytic layers have their carbon carriers different inion exchange capacity. It is noted that, relative to FIG. 6, likelocations are designated by like reference chanters, omitting thedescription.

FIG. 8 is an enlarged sectional view of a cathode catalytic layeraccording to the third embodiment. The cathode catalytic layer 10 isconfigured with a first catalytic layer 12 and a second catalytic layer13. The first catalytic layer 12 is composed of an amorphous carbon 21supporting Pt particles 20 thereon, with intervening electrolyte A. Onthe other hand, the second catalytic layer 13 is composed of ahigh-crystallinity carbon (graphitized Ketjenblack) 21 supporting Ptparticles 20 thereon, with intervening electrolyte B. The electrolyte Aand the electrolyte B have their ion exchange capacities (amounts ofprotons in the electrolytes) set to 0.9 meq/g and 1.2 meq/g,respectively, the electrolyte B having a greater ion exchange capacitythan the electrolyte A. The Pt support amount is set to 0.2 mg/cm² forthe first catalytic layer 12 and the second catalytic layer 13. Theamount of electrolyte is defined in terms of a mass ratio to Pt amount,and for the first catalytic layer 12, electrolyte A=1:1 mixture, and forthe second catalytic layer 13, electrolyte B=1:0.9 mixture. It is notedthat the ratio of electrolyte amount and Pt amount is an illustrativeexample, and not limited thereto. For electrolyte amount, the definitionis made to Pt amount, while the electrolyte amount may be defined to themass of carriers.

The second catalytic layer 13 has an increased tendency for corrosion byoxidation in comparison with the first catalytic layer 12, and the ionexchange capacity of electrolyte B in the second catalytic layer 13 isset greater relative to electrolyte A in the first catalytic layer 12.

FIG. 9 shows potential distributions in a sectional direction in avicinity of a cathode 4 in a start of a fuel cell. Using electrolyte Bin the second catalytic layer 13 allows for a suppressed potentialreduction (V2<V2′) of electrolyte in comparison with the case of usingelectrolyte A. Accordingly, in the second catalytic layer 13, corrosionof carbon carriers can be suppressed. In addition, the second catalyticlayer 13 may have a reduced mixing amount of electrolyte B, therebyallowing for a suppressed flooding in the cathode 4, in particular atthe first catalytic layer 12.

According to the present embodiment, first and second catalytic layershave different electrolytes, thereby allowing for a reduced voltage dropdue to a flooding, an enhanced power generation performance, and anexcellent durability of the fuel cell.

Fourth Embodiment Embodiment Example 8

For a fourth embodiment, the cathode catalytic layer is double-layered,and respective catalytic layers have their support amounts of catalystparticles different in between. It is noted that, relative to FIG. 6,like locations are designated by like reference chanters, omitting thedescription.

FIG. 10 is a sectional view of a cathode catalytic layer according tothe fourth embodiment. The cathode catalytic layer 10 is configured witha first catalytic layer 12 and a second catalytic layer 13. The firstcatalytic layer 12 is composed of an amorphous carbon 23 supportingPt—Co alloy particles 22 thereon, with intervening electrolyte 24,having a Pt support amount set to 0.2 mg/cm². On the other hand, thesecond catalytic layer 13 is composed of a high-crystallinity carbon(graphitized Ketjenblack) 26 supporting Pt particles 25 thereon, withintervening electrolyte 27, having a Pt support amount set to 0.3mg/cm², so that the Pt support amount in the second catalytic layer 13is greater in comparison with the first catalytic layer 12.

It is noted that here is taken an illustrative example using Pt—Co alloyparticles 22 and Pt particles 25, which constitutes no restriction tocatalyst particles, providing that catalyst particles in the secondcatalytic layer 13 have a higher oxidizing potential in comparison withcatalyst particles in the first catalytic layer 12.

The cathode catalytic layer has potential distributions similar to thedistributions described with reference to FIG. 7. Metallic catalyst (Pt)of the second catalytic layer 13 may have a greater oxidizing potentialin comparison with metallic catalyst (Pt—Co alloy) in the firstcatalytic layer 12, to thereby allow for an enhanced anti-corrosivenessin the second catalytic layer 13. Further, the second catalytic layer 13may have an increased Pt support amount in comparison with Pt supportamount of the first catalytic layer 12, to thereby allow for an enhancedanti-corrosiveness in the second catalytic layer 13.

According to the present embodiment, a double-layered cathode catalyticlayer has a varied support amount of metallic catalyst therein, therebyallowing a reduced voltage drop accompanied by a reduced catalyticactivity due to an oxidation of metallic catalyst, thus allowing for anenhanced durability of the fuel cell.

Fifth Embodiment (Embodiment Example 9

For a fifth embodiment, the cathode catalytic layer is double-layered inpart.

FIG. 11 is a sectional view of a membrane electrode assembly accordingto the fifth embodiment. A fuel gas a and an oxidant gas b are conductedin opposite directions, and the cathode catalytic layer 10 is locallydouble-layered in such a part that corresponds to a region opposing avicinity of an upstream of the oxidant gas b. An enlarged section of thedouble-layered part of cathode catalytic layer 10 is shown in FIG. 12.

The cathode catalytic layer 10 is configured with a first catalyticlayer 12 and a second catalytic layer 13, the second catalytic layer 13being shorter in length. The first catalytic layer 12 is composed of aKetjenblack 29 supporting Pt particles 28 thereon, with interveningelectrolyte 30. On the other hand, the second catalytic layer 13 iscomposed of an acetylene black 31 supporting Pt particles 28 thereon,with intervening electrolyte 32. The Pt support amount is set greater inthe second catalytic layer 13 than in the first catalytic layer 12, andPt particles in the second catalytic layer 13 have a smaller averageparticle size than Pt particles in the first catalytic layer 12. Forexample, Pt particles in the first catalytic layer 12 may have anaverage particle size within a range of 2 nm to 3 nm, and Pt particlesin the second catalytic layer 13 may have a greater average particlesize within a range of 3 nm to 5 nm.

The combination of carbon carriers using a Ketjenblack 29 and anacetylene black (CP-250) 31 in the carbon catalytic layers 12 and 13 isnot limited thereto, providing that carbon carriers in the secondcatalytic layer 13 have a higher oxidizing (corroding) potential orhigher anti-corrosiveness than carbon carriers in the first catalyticlayer 12.

It is noted that although the cathode catalytic layer 10 is partiallydouble-layered in FIG. 11, an entirety of the cathode catalytic layer 10may also be double-layered to be effective, as a matter of course. Inaddition, although the fuel gas a and the oxidant gas b are conducted inopposite directions in the example illustrated herein, which may bemodified to have an oxidant gas b conducted in an identical direction tothe fuel gas a.

With a lapse of long interval of time after a stop of the fuel cell, theanode and the cathode may be exposed to the air. In a start of the fuelcell, typically, an intentional purge operation is exercised by using aninactive gas. However, assuming no purge operations by an inactive gas,the fuel cell may be started with the anode and the cathode exposed toair, and a fuel gas a (e.g. hydrogen gas) may be introduced to theanode, when protons move in a solid polymer electrolyte membrane 2,which will be described with reference to FIG. 13.

In the start of the fuel cell, as the fuel gas a (hydrogen gas) isintroduced to the anode, the solid polymer electrolyte membrane 2 havedifferent flux of protons (H⁺) near an upstream of the fuel gas a andnear a downstream of the fuel gas a. Near the upstream of fuel gas a,protons (H⁺) move from the anode side to the cathode side, forming likeflux of protons to a start of the fuel cell. On the other hand, near thedownstream of fuel gas a, protons move from the cathode side to theanode side. It is noted that such movement of protons are disclosed inUnited States Patent USPAP2002/0076582.

FIG. 14 is a diagram showing potential distributions in a sectionaldirection in a vicinal region A of the cathode to a region opposing avicinity of the upstream of fuel gas a. Electrolyte has a potentialdistribution depending on flux of protons, as described with referenceto FIG. 7 in the second embodiment, and the electrolyte potential islower in the second electrode catalytic layer 13 than in the firstelectrode catalytic layer 12, giving the second electrode catalyticlayer 13 an increased tendency for corrosion by oxidation. Accordingly,carbon carriers in the second electrode catalyst layer 13 have anenhanced anti-corrosiveness in comparison with the first electrodecatalytic layer 12, and the Pt particles size is reduced to allow the Ptsupport amount to be increased, thereby allowing for an enhancedanti-corrosiveness of oxidizer electrode 4.

According to the present embodiment, by provision of a double-layeredelectrode catalytic layer in an oxidizer electrode in a region opposinga vicinity of an upstream of a fuel gas, this region is allowed to havea decreased tendency for corrosion by oxidation, even under a highpotential to be developed upon introduction of a hydrogen gas in a startof the fuel cell. As a result, the fuel cell is allowed to have anenhanced durability even in the case of a repetition of start and stopof the fuel cell.

Sixth Embodiment Embodiment Example 10

For a sixth embodiment, an improvement is provided to the membraneelectrode assembly shown in the fifth embodiment.

FIG. 15 is a sectional view of a membrane electrode assembly accordingto the sixth embodiment. There is a cathode catalytic layer 10double-layered in a region opposing a vicinity of an upstream of a fuelgas a at a cathode 4 side, and in a region vicinal to a downstream of anoxidant gas b. It is noted that although in FIG. 11, two-dimensionally,the fuel gas a and the oxidant gas b are introduced in oppositedirections, the flows fuel gas a and oxidant gas b are not limitedthereto, subject to a double-layered configuration of cathode catalyticlayer 10 in accordance with flow directions of the fuel gas a and theoxidant gas b to be introduced.

A first catalytic layer 12 is composed of a Ketjenblack 21 supporting Ptparticles thereon, with intervening electrolyte A, and on the otherhand, a second catalytic layer 13 is composed of a graphitizedKetjenblack supporting Pt particles thereon, with interveningelectrolyte B. The Pt support amount is set to be greater in the secondcatalytic layer 13 than in the first catalytic layer 12, and the ionexchange capacity of electrolyte (amount of protons in the electrolyte,unit (meq/g) is set to be greater for the electrolyte B than for theelectrolyte A. The amount of electrolyte is defined in terms of a massratio to Pt support amount, and for the first catalytic layer 12, Pt:electrolyte A=1:1, and for the second catalytic layer 13, Pt:electrolyte B=1:0.9. The ratio of electrolyte amount and Pt supportamount is an illustrative example, and not limited thereto. Forelectrolyte amount, the definition is made to Pt support amount, whilethe electrolyte amount may be defined to the mass of carriers.

FIG. 16 is a sectional view of a membrane electrode assembly, describingmovements of protons in a start of the fuel cell. Protons have adistribution of movements in the sectional direction of a solid polymerelectrolyte membrane 2, which is identical to a current densitydistribution. The current density distribution depends on the oxygenconcentration, i.e., the oxidant gas b's flow direction, and the degreeof proton movements becomes greater in the upstream of oxidant gas b incomparison with the downstream of oxidant gas b. Further, with respectto the sectional direction of the solid polymer electrolyte membrane 2,electrolyte has a potential distribution, which is decreased from theupstream of oxidant gas b toward the downstream of oxidant gas b, likethe degree of movements of protons. Further, due to fast movements ofelectrons, the oxidant gas b has a constant potential, whether in theupstream or downstream of oxidant gas b. As will be seen from suchpoints, the voltage (potential difference) V2 in the downstream ofoxidant gas b is greater than the voltage (potential difference) V1 inthe upstream of oxidant gas b. Therefore, in a vicinity of thedownstream of oxidant gas b, the cathode is exposed to an environmentwith an increased tendency for oxidation. However, the catalytic layer10 in the cathode is now double-layered in a vicinity of the downstreamof oxidant gas b, thus allowing for an enhanced anti-corrosiveness ofcathode. In regard of the deterioration in a start of the fuel cell, itis noted that description of the fifth embodiment is still applicable.

FIG. 17 is a diagram showing potential distributions in a sectionaldirection in a vicinal region B of cathode about the downstream ofoxidant gas b. The potential distribution of electrolyte is establishedin accordance with flux of protons, and the electrolyte potential islower in the second catalytic layer 13 than in the first catalytic layer12. Therefore, the ion exchange capacity of electrolyte B in the secondcatalytic layer 13 is increased relative to electrolyte A in the firstcatalytic layer 12, to thereby suppress the reduction of electrolytepotential, allowing for an enhanced anti-corrosiveness of cathode.

Further, according to the present embodiment, at the downstream ofoxidant gas b with an increased tendency for water to be residual, theamount of electrolyte B in the second catalytic layer 13 is decreasedrelative to electrolyte A in the first catalytic layer 12, to therebysuppress a flooding in the first catalytic layer 12.

Therefore, according to the present embodiment, by provision of alocally double-layered cathode catalytic layer, the anti-corrosivenessof cathode can be enhanced such as in introduction of a hydrogen gas ina start of the fuel cell or in power generation of the fuel cell,allowing for a suppressed high-potential state. As a result, the voltagedrop due to a flooding can be reduced, allowing for provision of a fuelcell excellent in power generation performance.

Further, Pt particles are employed as catalyst particles herein, whichare not limited to Pt particles, and may well be Ru, Rh, Pd, Ag, Ir, Pt,Au, or the like.

It is noted that although illustrative examples of double-layeredcathode catalytic layers are shown in the second embodiment to the sixthembodiment, the cathode catalytic layer in the proton-exchange membranefuel cell shown in the first embodiment may also be double-layered as amatter of course, to thereby allow for a yet enhanced anti-corrosivenessin the cathode catalytic layer.

Although favorable modes of embodiment of the present invention havebeen illustrated, the present invention is not restricted to those modesof embodiment, and it will be apparent that artisan may devise varietiesof other embodiment modes or modifications without departing from thescope of following claims.

INDUSTRIAL APPLICABILITY

According to the present invention, a polymer electrolyte fuel cell isallowed to prevent a corroding deterioration of carbon carriers in acathode catalytic layer in start and stop of the fuel cell, allowing foran enhanced stable output even in a running over a long term, with ahigh industrial applicability.

1. A polymer electrolyte fuel cell, comprising: a solid polymerelectrolyte membrane; catalytic layers disposed on both sides of thesolid polymer electrolyte membrane; gas diffusion layers disposedoutside the catalytic layers; and separators disposed outside the gasdiffusion layers, wherein a cathode-sided catalytic layer of thecatalytic layers comprises: a carbon carrier comprising carbon having amean lattice plane spacing d₀₀₂ of [002] planes calculated from an X-raydiffraction within a range of 0.343 nm to 0.358 nm, a crystallite sizeLc within a range of 3 nm to 10 nm; catalyst particles containingplatinum supported on the carbon carrier; and an electrolyte, whereinthe carbon carrier comprises a carbon black having a bulk density withina range of 0.09 g/cm³ to 0.13 g/cm³.
 2. The polymer electrolyte fuelcell as claimed in claim 1, wherein the carbon black has an electricalresistivity within a range of 0.27 Ωcm to 0.33 Ωcm.
 3. The polymerelectrolyte fuel cell as claimed in claim 1, wherein the carbon carriercomprises an acetylene black having a mean lattice plane spacing d₀₀₂ of[002] planes calculated from an X-ray diffraction within a range of0.343 nm to 0.355 nm, a crystallite size Lc within a range of 3 nm to 9nm, a bulk density within a range of 0.10 g/cm³ to 0.12 g/cm³, and anelectrical resistivity within a range of 0.29 Ωcm to 0.32 Ωcm.
 4. Thepolymer electrolyte fuel cell as claimed in claim 1, wherein thecatalyst particles occupy a proportion within a range of 30% to 70% in amass conversion with respect to a total amount of the catalyst particlesand the carbon carriers residing in the cathode catalytic layer, and thecatalyst particle-supporting carbon carrier has a specific surface areawithin a range of 60 m²/g to 200 m²/g.
 5. The polymer electrolyte fuelcell as claimed in claim 1, wherein the electrolyte in the cathode-sidedcatalytic layer and the solid polymer electrolyte membrane compriseperfluorocarbon polymers having sulfonic acid groups.
 6. The polymerelectrolyte fuel cell as claimed in claim 1, wherein the cathode-sidedcatalytic layer has an average thickness ranging 6 μm to 15 μm, and thecatalyst particle-supporting carbon carrier has a proportion ofexistence within a range of 50% to 80% with respect to a total mass inwhich the electrolyte and the catalyst particle-supporting carboncarrier are summed up.
 7. The polymer electrolyte fuel cell as claimedin claim 1, wherein, for an anode side, the catalytic layer has anaverage thickness ranging 2 μm to 10 μm, and the catalystparticle-supporting carbon carrier has a proportion of existence withina range of 50% to 80% with respect to a total mass in which theelectrolyte and the catalyst particle-supporting carbon carrier aresummed up.
 8. The polymer electrolyte fuel cell as claimed in claim 1,wherein, for an anode side, the catalytic layer has an average thicknessYa thinner than an average thickness Yc of the cathode-sided catalyticlayer.
 9. The polymer electrolyte fuel cell as claimed in claim 1,wherein, for an anode side, the catalytic layer has an average thicknessYa with a relationship of Ya/Yc=0.1 to 0.6 to an average thickness Yc ofthe cathode-sided catalytic layer.
 10. The polymer electrolyte fuel cellas claimed in claim 1, wherein the catalyst particles comprise aplatinum alloy containing a metal selected from the group consisting ofruthenium, rhodium, palladium, iridium, osmium, chromium, cobalt, andnickel.
 11. The polymer electrolyte fuel cell as claimed in claim 10,wherein the platinum alloy has a mixing ratio (platinum/metal) ofplatinum and the metal ranging 3/1 to 5/1 in a mole ratio.
 12. Thepolymer electrolyte fuel cell as claimed in claim 1, wherein, for ananode side, the catalytic layer comprises a carbon carrier having aspecific surface area within a range of 300 m²/g to 1,500 m²/g, catalystparticles containing platinum supported on the carbon carrier, and anelectrolyte.
 13. The polymer electrolyte fuel cell as claimed in claim1, wherein the cathode-sided catalytic layer comprises a first catalyticlayer and a second catalytic layer, and carbon carriers in the secondcatalytic layer have a higher anti-corrosiveness in comparison withcarbon carriers in the first catalytic layer neighboring the solidpolymer electrolyte membrane.
 14. The polymer electrolyte fuel cell asclaimed in claim 13, wherein an electrolyte in the second catalyticlayer has a greater ion exchange capacity in comparison with anelectrolyte in the first catalytic layer.
 15. The polymer electrolytefuel cell as claimed in claim 13, wherein the second catalytic layer hasa greater support amount of the catalyst particles therein in comparisonwith a support amount of the catalyst particles in the first catalyticlayer.
 16. The polymer electrolyte fuel cell as claimed in claim 1,wherein a double-layered catalytic layer of the cathode-sided catalyticlayer is disposed in a region opposing a vicinity of an upstream of afuel gas.
 17. The polymer electrolyte fuel cell as claimed in claim 1,wherein a double-layered catalytic layer of the cathode-sided catalyticlayer is disposed in a downstream region of an oxidant gas.